An attempt by the city's internal watchdog to verify how much money could be saved through a new garbage collection system touted by Mayor Rahm Emanuel failed because of a lack of cooperation from the administration, Inspector General Joseph Ferguson said Monday.

Ferguson late last year set out to audit the city's new grid-based garbage collection system, which eliminated the old method of picking up refuse on a ward-by-ward basis.

But three months into the audit, Streets and Sanitation Commissioner Charles Williams "abruptly ended (a) meeting and walked out" when asked to provide details on how savings were being measured, Ferguson stated in his report.

After that March 21 meeting, the Streets and Sanitation Department continued to withhold the details requested by Ferguson's office, the report stated. Ferguson eventually decided to cut short his effort because he was unable to evaluate the fundamental effects of the change, including "the number of trucks and personnel involved in the garbage collection process," the report stated.

Less than a month after the March meeting, Emanuel's administration put out a news release stating that the conversion to the grid-based system had been completed and would save $18 million a year that would be redirected to other services. Rollout of the grid system began in mid-2012.

The mayor's office stated that the city was deploying 40 fewer trucks each day while "using fewer crews and fuel." It stated that money also was being saved on tree planting, tree trimming and graffiti removal, which also had been switched to a grid system.

Ferguson recommended Streets and Sanitation "substantiate the reported $18 million savings by publicly releasing the data, calculations and supporting documentation that supports that public assertion."

The inspector general report also said that before the breakdown of cooperation, Streets and Sanitation officials conceded that the continued use of 50 ward supervisors to help oversee garbage collection "did not meet current management needs" but that officials "blocked (Ferguson's) effort to review or validate management's claimed efforts to correct his self-identified deficiency."

By keeping in place ward supervisors, who largely report to aldermen, Emanuel helped ensure the backing of the City Council for the change to the grid-based system, which aldermen had opposed for years, fearing they would lose their control of an essential city service.

In an email response to Ferguson's report, the Streets and Sanitation Department reiterated that it expects the grid-based system to save $18 million a year and said Ferguson should have waited until the rollout of the new system was complete before conducting an audit — even though the intent before full cooperation was cut off was to complete the audit in late summer, when sufficient data were available.

"We believe the inspector general's audit of the system midway through implementation was premature, and the appropriate approach to auditing the large scale transition of Chicago's refuse collection system is to roll the program out completely, review for issues and best practices, adjust strategies accordingly, and audit," Williams said in the statement.

The inspector general's report comes less than two months after Ferguson concluded that City Hall cannot back up claims that its controversial red-light camera program is designed to make intersections safer. The city could not provide documents to prove the cameras went up at intersections with the most side-impact crashes, which proponents say the cameras help prevent.

The reports are a sign of mounting tension between Emanuel and Ferguson, whose term expires in late November. The administration recently won a state Supreme Court ruling that blocks the inspector general from independently enforcing subpoenas of City Hall documents.